use a single Go routine to send copies of CONNECTION_CLOSE packets

[marten-seemann]

Fixes #3477. The design is definitely a lot cleaner than what we had before.

This will drastically reduce the number of active Go routines, especially when handling a lot of short-lived QUIC connection.

I'm also happy to have found a use case for the math/bits package.

@Stebalien, can you take a look at this PR?

[codecov]

Codecov Report

Base: 85.70% // Head: 85.61% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (263f728) compared to base (57650fc).
Patch coverage: 68.57% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3514      +/-   ##
==========================================
- Coverage   85.70%   85.61%   -0.08%     
==========================================
  Files         136      137       +1     
  Lines       10004    10001       -3     
==========================================
- Hits         8573     8562      -11     
- Misses       1053     1062       +9     
+ Partials      378      377       -1     

Impacted Files	Coverage Δ
zero_rtt_queue.go	57.14% <57.14%> (ø)
packet_handler_map.go	72.37% <60.00%> (-1.73%)	⬇️
closed_conn.go	64.71% <91.67%> (-14.71%)	⬇️
conn_id_generator.go	89.66% <100.00%> (+0.37%)	⬆️
connection.go	77.69% <100.00%> (-0.02%)	⬇️

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